This invention relates to mobile (or portable) cellular communication systems, and more particularly to an antenna apparatus for use by mobile subscriber units to provide beam forming transmission and reception capabilities.
Code division multiple access (CDMA) communication systems provide wireless communications between a base station and one or more mobile subscriber units. The base station is typically a computer controlled set of transceivers that are interconnected to a land-based public switched telephone network (PSTN). The base station includes an antenna apparatus for sending forward link radio frequency signals to the mobile subscriber units. The base station antenna also receives reverse link radio frequency signals transmitted from each mobile unit. Each mobile subscriber unit also contains an antenna apparatus for the reception of the forward link signals and for transmission of the reverse links signals. A typical mobile subscriber unit is a digital cellular telephone handset or a personal computer coupled to a cellular modem. In CDMA cellular systems, multiple mobile subscriber units may transmit and receive signals on the same frequency, but with different modulation codes, to distinguish signals sent to or received from individual subscriber units.
The most common type of antenna for transmitting and receiving signals at a mobile subscriber unit is a monopole or omnidirectional antenna. This type of antenna consists of a single wire or antenna element that is coupled to a transceiver within the subscriber unit. The transceiver receives reverse link signals to be transmitted from circuitry within the subscriber unit and modulates the signals onto a carrier signal at a specific frequency assigned to that subscriber unit. The modulated carrier signal is transmitted by the antenna element. Forward link signals received by the antenna element at a specific frequency are demodulated by the transceiver and supplied to processing circuitry within the subscriber unit.
The signal transmitted from a monopole antenna is omnidirectional in nature. That is, the signal is sent with the same signal strength in all directions in a generally horizontal plane. Reception of a signal with a monopole antenna element is likewise omnidirectional. A monopole antenna does not differentiate in its ability to detect a signal in one direction versus detection of the same or a different signal coming from another direction. Generally, a monopole antenna does not produce significant radiation in the azimuth direction. The antenna pattern is commonly referred to as a donut shape with the antenna element located at the center of the donut hole.
A second type of antenna that may be used by mobile subscriber units is described in U.S. Pat. No. 5,617,102. The system described therein provides a directional antenna comprising two antenna elements mounted on the outer case of a laptop computer, for example. The system includes a phase shifter attached to each element. The phase shifter may be switched on or off to effect the phase of signals transmitted or received during communications to and from the computer. By switching the phase shifters on and regulating the amount of phase shift imparted to the signals input thereto, the antenna pattern (which applies to both the receive and transmit modes) may be modified to provide a concentrated signal or beam in the selected direction. This is referred to as an increase in antenna gain or directionality. The dual element antenna of the cited patent thereby directs the transmitted signal into predetermined quadrants or directions to allow for changes in orientation of the subscriber unit relative to the base station, while minimizing signal loss due to the orientation change. In accordance with the antenna reciprocity theorem, the antenna receive characteristics are similarly effected by the use of the phase shifters.
CDMA cellular systems are also recognized as being interference limited systems. That is, as more mobile subscriber units become active in a cell and in adjacent cells, frequency interference becomes greater and thus error rates increase. As error rates increase, to maintain signal and system integrity, the operator must decrease the maximum data rates allowable. Thus, another method by which data rate can be increased in a CDMA system is to decrease the number of active mobile subscriber units, thus clearing the airwaves of potential interference. For instance, to increase the maximum available data rate by a factor of two, the number of active mobile subscriber units can be decreased by one half. However, this is rarely an effective mechanism to increase data rates due to the lack of priority assignments to the system users.
Problems of the Prior Art
Various problems are inherent in prior art antennas used on mobile subscriber units in wireless communications systems. One such problem is called multipath fading. In multipath fading, a radio frequency signal transmitted from a sender (either a base station or mobile subscriber unit) may encounter interference on route to the intended receiver. The signal may, for example, be reflected from objects, such as buildings that are not in the direct path of transmission, but that redirect a reflected version of the original signal to the receiver. In such instances, the receiver receives two versions of the same radio signal; the original version and a reflected version. Each received signal is at the same frequency, but the reflected signal may be out of phase with the original due to the reflection and consequent longer transmission path. As a result, the original and reflected signals may partially cancel each other out (destructive interference), resulting in fading or dropouts in the received signal, hence the term multipath fading.
Single element antennas are highly susceptible to multipath fading. A single element antenna has no way of determining the direction from which a transmitted signal is sent and cannot be tuned or attenuated to more accurately detect and receive a signal in any particular direction. Its directional pattern is fixed by the physical structure of the antenna components.
The dual element antenna described in the aforementioned reference is also susceptible to multipath fading, due to the symmetrical and opposing nature of the hemispherical lobes formed by the antenna pattern when the phase shifter is activated. Since the lobes created in the antenna pattern are more or less symmetrical and opposite from one another, a signal reflected in a reverse direction from its origin can be received with as much power as the original signal that is received directly. That is, if the original signal reflects from an object beyond or behind the intended receiver (with respect to the sender) and reflects back at the intended receiver from the opposite direction as the directly received signal, a phase difference in the two signals can create destructive interference due to multipath fading.
Another problem present in cellular communication systems is inter-cell signal interference. Most cellular systems are divided into individual cells, with each cell having a base station located at its center. The placement of each base station is arranged such that neighboring base stations are located at approximately sixty degree intervals from each other. In essence, each cell may be viewed as a six sided polygon with a base station at the center. The edges of each cell adjoin each other and a group of cells form a honeycomb-like image if each cell edge were to be drawn as a line and all cells were viewed from above. The distance from the edge of a cell to its base station is typically driven by the maximum amount of power that is to be required to transmit an acceptable signal from a mobile subscriber unit located near the edge of the cell to that cell""s base station (i.e., the power required to transmit an acceptable signal a distance equal to the radius of one cell).
Intercell interference occurs when a mobile subscriber unit near the edge of one cell transmits a signal that crosses over the edge into a neighboring cell and interferes with communications taking place within the neighboring cell. Typically, intercell interference occurs when similar frequencies are used for communications in neighboring cells. The problem of intercell interference is compounded by the fact that subscriber units near the edges of a cell typically use higher transmit powers so that the signals they transmit can be effectively received by the intended base station located at the cell center. Consider that the signal from another mobile subscriber unit located beyond or behind the intended receiver may be arrive at the base station at the same power level, representing additional interference.
The intercell interference problem is exacerbated in CDMA systems, since the subscriber units in adjacent cells may typically be transmitting on the same frequency. For example, generally, two subscriber units in adjacent cells operating at the same carrier frequency but transmitting to different base stations will interfere with each other if both signals are received at one of the base stations. One signal appears as noise relative to the other. The degree of interference and the receiver""s ability to detect and demodulate the intended signal is also influenced by the power level at which the subscriber units are operating. If one of the subscriber units is situated at the edge of a cell, it transmits at a higher power level, relative to other units within its cell and the adjacent cell, to reach the intended base station. But, its signal is also received by the unintended base station, i.e., the base station in the adjacent cell. Depending on the relative power level of two same-carrier frequency signals received at the unintended base station, it may not be able to properly identify a signal transmitted from within its cell from the signal transmitted from the adjacent cell. What is needed is a way to reduce the subscriber unit antenna""s apparent field of view, which can have a marked effect on the operation of the forward link (base to subscriber) by reducing the apparent number of interfering transmissions received at a base station. A similar improvement is needed for the reverse link, so that the transmitted signal power needed to achieve a particular receive signal quality can be reduced.
The present invention provides an inexpensive antenna apparatus for use with a mobile or portable subscriber unit in a wireless same-frequency communications system, such as a CDMA cellular communications system.
The invention provides a mechanism and method for efficiently configuring the antenna apparatus to maximize the effective radiated and/or received energy. The antenna apparatus includes multiple antenna elements and a like number of adjustable weight control components. As is well known in the art, the weight control components are controllable to adjust the phase, amplitude and/or delay of the signal coupled to each of the antenna elements. The weight control components (e.g., phase shifter, delay line, amplifier with variable gain, switch) are thus jointly and independently operable to affect the direction of reverse link signals transmitted from the subscriber unit on each of the antenna elements and the direction of forward link signals transmitted to the subscriber unit.
It is well known to steer or adapt an antenna that comprises a plurality of elements to maximize a given signal quality metric, such as the signal to interference plus noise ratio (SINR). The array is steered or directed by changing the relative phase angle or amplitude (i.e., weight) between the signals input to each of the antenna elements. Typically, the antenna is adapted or steered to achieve a maximum signal quality metric while operating in the receive mode or to steer the beam to a selected direction for transmitting.
According to the teachings of the present invention, the transmit beam pattern from a transmitter (or transceiver) is optimized to achieve an optimal signal quality metric at a receiver (or transceiver). That is, the transmit beam is adapted or steered dependent on the signal received at the receiver. This approach is substantially different from the prior art that teaches adapting an antenna of a receiver in response to the received quality metric. According to the present invention, a first station transmits to a second station where a signal quality metric is measured. The first station scans a plurality of antenna directional angles and the second station measures the signal quality metric at each directional angle. The second station then communicates the signal quality metric information back to the first station where the optimum signal quality metric value is selected. This selected value is then correlated with the antenna directional angle that produced it and the antenna is steered to that directional angle for communicating with the second station. In lieu of sending the signal quality metric information for each directional angle, the second station can choose the optimum signal quality metric and transmit only the optimum value back to the first station. The second station can also transmit differential signal quality metric information in lieu of the absolute signal quality metric values. Also, the signal quality metric information can be sent to the first station as each value is determined or the values can be stored and sent later as a group. The second station can further check each of the signal quality metrics against a predetermined threshold and transmit back to the first station only those signal quality metric values that exceed the threshold.
Through the use of an array of antenna elements, each having a programmable weight control component for forming the antenna beam as desired, the antenna apparatus increases the effective transmit power per bit transmitted. Thus, the number of active subscriber units in a cell may remain the same while the antenna apparatus of this invention increases data rates for each subscriber unit beyond those achievable by prior art antennas. Alternatively, if data rates are maintained at a given rate, more subscriber units may become simultaneously active in a single cell using the antenna apparatus described herein. In either case, the capacity of a cell is increased, as measured by the sum total of data being communicated at any given time.
Forward link communications capacity can be increased as well, due to the directional reception capabilities of the antenna apparatus. Since the antenna apparatus is less susceptible to interference from adjacent cells, the forward link system capacity can be increased by adding more users or by increasing the cell radius.
With respect to the physical implementation of the antenna apparatus, one embodiment of the invention specifies that first, second, and third antenna elements are positioned at locations corresponding to corners of an equilateral triangle and are aligned orthogonal to a plane defined by the triangle. Other embodiments specify that first, second, third, and fourth antenna elements are positioned at locations corresponding to corners of a rectangle or square, with the fifth antenna element positioned at a location corresponding to a center of the rectangle or square.